Neural function requires accurate control of gene transcription in response to environmental stimuli. Epigenetics plays a crucial role in this process by modulating gene expression without changing underlying DNA sequences. DNA methylation is a major epigenetic mechanism, wherein a methyl group is covalently coupled to the C5 position of cytosine, predominantly at 5'-CpG-3' dinucleotide sites. Though still few in number, accumulating evidence demonstrates that DNA methylation may play important roles in drug addiction. However, to firmly establish DNA methylation's role in drug addiction, the question of whether and how DNA methylation changes take place and function in neurons must be addressed. This is particularly true for those defined neuronal subtypes selectively engaged in drug addiction. To address this question, we herein propose to study methylcytosine dioxygenase TET1, a newly identified DNA demethylation enzyme, and its mediated DNA methylation turnover in the two major neuron types (D1- and D2- medium spiny neurons (MSNs)) in nucleus accumbens in cocaine addiction. In the past, we have found TET1, but not TET2 or TET3 (the other two members of the dioxygenase family), is selectively regulated by cocaine in the nucleus accumbens. We have also shown TET1 mediates drug reward behaviors. Here, we propose TET1 carries distinct functions in D1- and D2-MSNs, which usually play opposite roles in addiction reward circuitry. We will also elucidate TET1-mediated neuron subtype-specific DNA methylation changes in D1- and D2-MSNs. Lastly, we will probe the long-standing question of the causal functional role of DNA methylation in drug addiction-relevant behavior through a proven CRISPR-Cas9 based epigenome editing approach to modify DNA methylation at selective loci. Upon completion, our study will not only advance our scientific understanding of DNA epigenetic underpinnings of drug addiction in a neuron subtype-specific manner, it will also provide a path to manipulate behaviors associated with drug addiction through cell type- specific precision epigenome editing, which has obvious potential utility for future therapeutic applications.